1. Field of the Invention
The present invention relates to technology for effective use of an aggregation scheme suggested to improve a throughput in an Institute of Electrical and Electronics Engineers (IEEE) 802.11n and an IEEE 802.11ac.
2. Description of the Related Art
An Institute of Electrical and Electronics Engineers (IEEE) 802.11 standardization group providing a current wireless local area network (WLAN) standard has been establishing a next generation standard in a direction of increasing a speed of a physical (PHY) layer in accordance with advancements in technology.
Currently, a PHY layer speed may be supported by up to 11 megabits per second (Mbps) in 802.11b, up to 54 Mbps in 802.11g and 802.11a, up to 150 Mbps based on a single transmission and reception antenna in 802.11n, and up to 300 Mbps based on a single transmission and reception antenna in 802.11ac having a process of standardization.
However, a speed increment in a media access control (MAC) layer may not be directly proportional to a speed increment in the PHY layer. A speed experienced by a user may correspond to a speed of the MAC layer and thus, the speed increment in the PHY layer may not provide an increase in the speed experienced by the user.
Such an issue may be caused by MAC header information, preamble information included for each packet, and an overhead occurring due to an ACK transmission performed after a short interframe space (SIFS), for example, 16 microseconds (μs).
To solve an issue of inefficiency resulting from the overhead, a MAC layer of an 802.11n standard defines two aggregation methods including an aggregated-MAC service data unit (A-MSDU) and an aggregated-MAC protocol data unit (A-MPDU). The two aggregation methods may be utilized to improve an efficiency of a MAC throughput in a case in which the PHY layer speed is greater than 1 gigabit per second (Gbps) such as a case of 802.11ac.
FIG. 1 is a diagram illustrating a configuration of a conventional A-MSDU frame.
Referring to FIG. 1, a single A-MSDU may include one PHY preamble, one PHY header, and one MAC header. Also, a plurality of packets may be distinguished by a 14 bytes delimiter including a destination address (DA), a source address (SA), and a length.
FIG. 2 is a diagram illustrating a configuration of a conventional A-MPDU frame.
Referring to FIGS. 2 and 3, each subframe may include an MPDU including a MAC header, and a 4 byte delimiter having a packet to which cyclic redundancy check (CRC) information is incorporated.
FIG. 3 is a diagram illustrating a configuration of a conventional A-MPDU subframe.
Effects of using an A-MSDU differ from Effects of using an A-MPDU. Since, the A-MSDU includes one MAC header and one PHY header irrespective of a number of MSDUs to be connected, a ratio of an overhead may be relatively small when compared to an entire packet. Thus, when a channel state is favorable and an error does not occur, using the A-MSDU may provide a maximal efficiency.
However, in a wireless channel, a packet error rate may increase based on a distance due to characteristics of the wireless channel. Transmission using a scheme having a high reliability level, which may not lead to an error, may require a relatively low PHY rate. In this case, effects of aggregation may not be realized.
When the A-MSDU is used in an error channel, the A-MSDU may need to be entirely retransmitted although an error occurs in one bit of the MSDUs included in the A-MSDU because an error MSDU of the MSDUs included in the A-MSDU may not be recognized.
Thus, in a case of requiring retransmission as described above, using the A-MSDU may be unsuitable.
In a standard, the A-MSDU is defined to be used by selecting one from two maximum lengths of 4 kilobytes (kB) and 8 kB. In general, commercial products adopt 4 kB as a maximum length. When aggregation is performed based on 8 kB, an error rate of a packet may be increased as compared to 4 kB. Thus, effects resulting from the aggregation may not be realized.
Accordingly, most commercial products may adopt the A-MPDU method, in lieu of the A-MSDU method.
In a case of the A-MPDU, each of the packets included in the A-MPDU may include the MAC header information, and the CRC information associated with a corresponding packet may be included in a tail of each of the packets. Thus, when compared to the A-MSDU, the overhead may be generated by incorporating the MAC header information for each of the packets.
However, when an error occurs in a predetermined portion in a wireless state including an error channel, the A-MPDU may select another packet, aside from the packet including an error portion based on the CRC information included in each of the packets, thereby receiving the selected packet. Thus, the packet including the error portion may be selectively retransmitted rather than transmitting whole packets. In this light, the standard allows transmission of up to 64 kB in a process of configuring the A-MPDU in contrast to the A-MSDU in which the maximum length to be transmitted is limited by up to 8 kB.
Currently, the IEEE 802.11n standard defines only a configuration of the A-MPDU. Although the maximum length is limited up to 64 kB or a number of packets is limited up to 64, a method of configuring the A-MPDU has not been defined.
Thus, when the number of packets configuring the A-MPDU is fixed to be a predetermined number, efficiency of the throughput may be reduced and a delay time in a process may become longer.
As the number of packets configuring the A-MPDU increases, channel use efficiency may be increased and thus, the MAC throughput may be improved. However, a delay for generating the packet and transmitting the generated packet may become longer because a packet arriving at first may need to standby until a number of packets required for configuring the A-MPDU is stacked. Accordingly, a trade-off relationship may be established between the delay and the number of packets for configuring the A-MPDA, for example, the throughput.
Such delay may not affect a state in which a speed of generating a packet is greater than a speed of processing the PHY layer. When the speed of generating a packet is greater than the speed of processing the PHY layer, a packet queue of a MAC may be full of packets. In this instance, a maximum number of packets may be aggregated for transmission.